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1 /*
2  * Copyright (c) 2013 Damien Miller <djm@mindrot.org>
3  *
4  * Permission to use, copy, modify, and distribute this software for any
5  * purpose with or without fee is hereby granted, provided that the above
6  * copyright notice and this permission notice appear in all copies.
7  *
8  * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
9  * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
10  * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
11  * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
12  * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
13  * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
14  * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
15  */
16 
17 /* $OpenBSD: cipher-chachapoly.c,v 1.8 2016/08/03 05:41:57 djm Exp $ */
18 
19 #include "includes.h"
20 
21 #include <sys/types.h>
22 #include <stdarg.h> /* needed for log.h */
23 #include <string.h>
24 #include <stdio.h>  /* needed for misc.h */
25 
26 #include "log.h"
27 #include "sshbuf.h"
28 #include "ssherr.h"
29 #include "cipher-chachapoly.h"
30 
31 int
chachapoly_init(struct chachapoly_ctx * ctx,const u_char * key,u_int keylen)32 chachapoly_init(struct chachapoly_ctx *ctx,
33     const u_char *key, u_int keylen)
34 {
35 	if (keylen != (32 + 32)) /* 2 x 256 bit keys */
36 		return SSH_ERR_INVALID_ARGUMENT;
37 	chacha_keysetup(&ctx->main_ctx, key, 256);
38 	chacha_keysetup(&ctx->header_ctx, key + 32, 256);
39 	return 0;
40 }
41 
42 /*
43  * chachapoly_crypt() operates as following:
44  * En/decrypt with header key 'aadlen' bytes from 'src', storing result
45  * to 'dest'. The ciphertext here is treated as additional authenticated
46  * data for MAC calculation.
47  * En/decrypt 'len' bytes at offset 'aadlen' from 'src' to 'dest'. Use
48  * POLY1305_TAGLEN bytes at offset 'len'+'aadlen' as the authentication
49  * tag. This tag is written on encryption and verified on decryption.
50  */
51 int
chachapoly_crypt(struct chachapoly_ctx * ctx,u_int seqnr,u_char * dest,const u_char * src,u_int len,u_int aadlen,u_int authlen,int do_encrypt)52 chachapoly_crypt(struct chachapoly_ctx *ctx, u_int seqnr, u_char *dest,
53     const u_char *src, u_int len, u_int aadlen, u_int authlen, int do_encrypt)
54 {
55 	u_char seqbuf[8];
56 	const u_char one[8] = { 1, 0, 0, 0, 0, 0, 0, 0 }; /* NB little-endian */
57 	u_char expected_tag[POLY1305_TAGLEN], poly_key[POLY1305_KEYLEN];
58 	int r = SSH_ERR_INTERNAL_ERROR;
59 
60 	/*
61 	 * Run ChaCha20 once to generate the Poly1305 key. The IV is the
62 	 * packet sequence number.
63 	 */
64 	memset(poly_key, 0, sizeof(poly_key));
65 	POKE_U64(seqbuf, seqnr);
66 	chacha_ivsetup(&ctx->main_ctx, seqbuf, NULL);
67 	chacha_encrypt_bytes(&ctx->main_ctx,
68 	    poly_key, poly_key, sizeof(poly_key));
69 
70 	/* If decrypting, check tag before anything else */
71 	if (!do_encrypt) {
72 		const u_char *tag = src + aadlen + len;
73 
74 		poly1305_auth(expected_tag, src, aadlen + len, poly_key);
75 		if (timingsafe_bcmp(expected_tag, tag, POLY1305_TAGLEN) != 0) {
76 			r = SSH_ERR_MAC_INVALID;
77 			goto out;
78 		}
79 	}
80 
81 	/* Crypt additional data */
82 	if (aadlen) {
83 		chacha_ivsetup(&ctx->header_ctx, seqbuf, NULL);
84 		chacha_encrypt_bytes(&ctx->header_ctx, src, dest, aadlen);
85 	}
86 
87 	/* Set Chacha's block counter to 1 */
88 	chacha_ivsetup(&ctx->main_ctx, seqbuf, one);
89 	chacha_encrypt_bytes(&ctx->main_ctx, src + aadlen,
90 	    dest + aadlen, len);
91 
92 	/* If encrypting, calculate and append tag */
93 	if (do_encrypt) {
94 		poly1305_auth(dest + aadlen + len, dest, aadlen + len,
95 		    poly_key);
96 	}
97 	r = 0;
98  out:
99 	explicit_bzero(expected_tag, sizeof(expected_tag));
100 	explicit_bzero(seqbuf, sizeof(seqbuf));
101 	explicit_bzero(poly_key, sizeof(poly_key));
102 	return r;
103 }
104 
105 /* Decrypt and extract the encrypted packet length */
106 int
chachapoly_get_length(struct chachapoly_ctx * ctx,u_int * plenp,u_int seqnr,const u_char * cp,u_int len)107 chachapoly_get_length(struct chachapoly_ctx *ctx,
108     u_int *plenp, u_int seqnr, const u_char *cp, u_int len)
109 {
110 	u_char buf[4], seqbuf[8];
111 
112 	if (len < 4)
113 		return SSH_ERR_MESSAGE_INCOMPLETE;
114 	POKE_U64(seqbuf, seqnr);
115 	chacha_ivsetup(&ctx->header_ctx, seqbuf, NULL);
116 	chacha_encrypt_bytes(&ctx->header_ctx, cp, buf, 4);
117 	*plenp = PEEK_U32(buf);
118 	return 0;
119 }
120